I feel like I'm retyping things I've said too many times before, but.....
1.
It's hard to be specific because of just how varied the views are of people who claim to be libertarian. On the soft end, maybe they mostly just want government to stop telling them what they can put in or do with their bodies. On the extreme end, it's virtually indistinguishable from anarchism.
When you look a the party platform, some ideas would be national suicide. Like the national tax policy:
When you pay taxes, do you do so voluntarily? Or do you do so because you are forced to do so? If you don’t pay your taxes, what will happen? Will you be fined further? Harassed by the IRS or other government entities? Jailed? The Libertarian Party is fundamentally opposed to the use of force […]
www.lp.org
No taxes, only voluntary donations to the federal government. Seriously. They wrote that. Just read it. So to the extent that kind of thinking is libertarianism, I would say we need as little of it as possible. Put that in law and you would see the immediate collapse of the federal government and everything related to it (like the dollar). Global catastrophe, but necessarily worse for those on the land...
Best thing you can say about that is that it probably wouldn't ever pass congress. You'll have to forgive me if I do not find that to be the most compelling argument in favor of an ideology. There other platform statements are similarly unwise.
On a local scale, here's what happens when you put various 'big L Libertarianism' ideas into practice:
When a group of libertarians set about scrapping their local government, chaos descended. And then the bears moved in.
newrepublic.com
Seriously, this happened. You should absolutely read about it.
www.vox.com
Roads falling apart, and basically a bear invasion because that's what you get when you have a town of people who get rid of their trash however they damn please without mean old government telling them what to do with it.
So as for specific policies and proposals, I haven't seen much of anything that makes me think libertarianism is a good idea. In practice, it seems to lead to societal failure. And the proposals that have never been put into practice seem to be dangerously naive.